EUTECTIC RESEARCH: THE ALLOYS OF LEAD AND TIN. 113 



the conclusion is justified that the duration of the exposure to liquid air was not long 

 enough to allow this transformation to occur. 



The comparison of the micro-structure of the specimens quenched from a tempe- 

 rature just below the recalescence in question with that of very slowly- cooled 

 specimens also shows only very slight differences, so that the conclusion is justified 

 that the differences observed between the structure of specimens quenched above the 

 recalescence temperature and those which have been allowed to undergo the change 

 unhindered are due entirely to the partial suppression of the transformation associated 

 with these evolutions of heat. 



The alloys chosen for close study in this respect were those containing 16 to 18 per 

 cent, of tin, since in this range the heat-evolution reaches its maximum intensity. A 

 high degree of magnification is required to bring out clearly the difference in micro- 

 structure between the alloys which have been quenched above the transformation 

 point and those which have been allowed to undergo the change. (See Plate 7.) 



Fig. 25 is a photo-micrograph (magnification 1000 diameters) of a specimen of an 

 alloy containing 16 per cent, of tin which has been kept at 175 0. for six weeks, and 

 has then been quenched in liquid air from a temperature of 160 C. Seen under a 

 low magnification this specimen appears practically homogeneous, but the high power 

 used in the photograph reveals the presence of a large number of very fine white 

 patches; from their behaviour and appearance these undoubtedly consist of free tin, 

 but their minute size and their distribution over the specimen, as well as the entire 

 absence of any spots or markings within the white patches, indicate that this tin is of 

 " secondary" origin, having been separated from the solid solution during cooling. To 

 some extent the presence of this tin in the quenched specimen is probably to be 

 ascribed to the fact that the rate of cooling employed was not great enough to 

 entirely suppress the change occurring at the recalescence. Figs. 26 and 27 are 

 photo-micrographs of a piece of the same ingot as that represented in the previous 

 figure, but in this case the specimen has not been quenched, but has undergone very 

 gradual cooling from 150 C. to the ordinary temperature. The mottling of secondary 

 tin is much more pronounced in these specimens, the quantity of tin being much 

 larger, while it is also aggregated into larger masses. Since the appearance of a 

 specimen quenched from a temperature just below the recalescence is very similar* to 

 that of this specimen, it follows that the transformation in question has involved the 

 rejection of a considerable quantity of tin from the solid solution. 



Figs. 28 and 29 respectively represent typical sections, under a magnification of 

 1000 diameters, of quenched and slowly-cooled alloys containing 18 per cent, of tin. 

 In the quenched specimen (fig. 28) we have two large masses of primary tin (remnants 

 of eutectic) surrounded by the ground-mass of dark solid solution mottled with minute 

 patches of secondary tin ; in the slowly-cooled specimen (fig. 29) the quantity of 



* The laminae of secondary tin, seen in fig. 27, are, however, only found in very slowly-cooled, 

 specimens they evidently result from aggregation during this process. 

 VOL. CCIX. A, O 



